We do not yet fully understand how host cells recognize and respond to viral infections, and our broad goal is to1 clarify mechanisms of anti-viral innate immunity in mammals. We have previously studied microbial components recognized by Toll-like receptors (TLRs) and established mouse models lacking each TLR. These studies revealed that viruses are recognized by TLR-dependent and -independent mechanisms and that recognition leads to induction of type I interferons (IFNs), cytokines critical for antiviral host defense in mammals. Although TLR signaling pathways leading to IFN production have been extensively studied, the molecular mechanisms of TLR-independent viral detection have remained poorly understood. Recently, we have focused on a family of potential cytoplasmic viral detectors and their signaling intermediates. We also have reason to believe that IFN-independent anti-viral host defense mechanisms operate in mammals, although we do not know precisely what these mechanisms might entail. In the present proposal, we plan to utilize mouse reverse genetics (gene targeting) to generate and analyzing mutant mice lacking molecules that have a strong likelihood of participating in antiviral responses. Combining targeted mutations with one another and using classical methods of pathway analysis, we hope to understand the how the presence of viruses is sensed, how signals are initiated, and how they are transduced to yield a defensive response. First, we will continue our studies of potential intracellular viral detectors and their signaling molecules leading to the production of type I IFNs and proinflammatory cytokines. Second, we will search for new candidate molecules potentially involved in signaling pathways that lead from from cytoplasmic viral detectors to permit type I IFN production. Toward this end, we will use expression cloning and yeast two-hybrid screening. Third, we will try to understand and characterize novel anti-viral host defense mechanisms. To do so we will rely upon data from the forward genetic initiatives pursued in Project 1 and Project 2. We will generate mice with mutations in homologues of candidate molecules identified by Drosophila forward genetics conducted by the Strasbourg group. We will also target paralogues of molecules identified by mouse forward genetics conducted by the La Jolla group. By comparing the phenotypes of these mice with those bearing mutations affecting pathways serving the TLRs or cytoplasmic viral detectors, we will develop a comprehensive understanding of the host response. The successful attainment of such an understanding is enormously important, since viral infections are one of the most important causes of death worldwide. Potentially, the new concepts that we develop may be applied to the improvement of anti-viral vaccines and to the development of small molecules that enforce anti-viral responses through interaction with newly identified molecules.